The present invention relates to a highly active water gas shift catalyst and a process for producing it, and also a process for converting a gas mixture comprising at least carbon monoxide and water into hydrogen and carbon dioxide in a wide temperature range using this catalyst.
In a fuel cell, electric energy is obtained by means of chemical reaction. Most fuel cells utilize the reaction of a reducing stream with an oxidizing stream, usually hydrogen and oxygen. To make a fuel usable in a fuel cell, this has to be converted beforehand into a hydrogen-rich stream.
The preliminary processing of fuels is often carried out in three steps:
The fuel is firstly reformed and in this way dissociated into CO and H2. This is followed by a water gas shift stage in which the CO formed is reacted with water in a temperature-dependent equilibrium reaction to give CO2 and H2:CO+H2O→CO2+H2 
This equilibrium lies more to the side of H2 and CO2, the lower the temperature. A CO fine purification stage usually follows.
High concentrations (greater than 50 ppm) of CO damage the anode of the fuel cells. The CO content therefore has to be minimized before the actual cell. This is carried out in the water gas shift stage and also in the CO fine purification stage. The water gas shift stage usually occurs in two temperature stages. A reaction at temperatures in the range from 150° C. to 280° C. is referred to as a low-temperature shift reaction (LTS). The LTS is usually carried out catalytically using Cu/Zn oxide catalysts. In the range from 280° C. to 550° C., the reaction is referred to as a high-temperature shift reaction (HTS). This is traditionally carried out over Fe/Cr catalysts. This reaction can also be catalyzed by Mo, Ni and further elements. Noble metals on cerium oxides have likewise been described a number of times as catalysts for this reaction.
The shift reaction not only leads to removal of the catalyst poison CO but also increases the proportion of the desired product H2 in the fuel stream. It is therefore important that a catalyst for the HTS catalyzes the production of H2 from CO and H2O but does not catalyze reactions which lead to elimination or depletion of the desired product H2. Such reactions include, in particular, methanation which can be observed over nickel catalysts at high temperatures and over noble metal catalysts even at temperatures above 350° C. This involves two reaction paths:CO+3H2→CH4+H2OCO2+4H2→CH4+2H2O
Both reactions consume the desired product H2 and therefore reduce the hydrogen yield.
Processes and catalysts which give a very high yield of hydrogen and display a very low tendency for methanation to occur are known from the prior art.
EP 1 571 125 A2 discloses a catalyst for separating carbon monoxide from hydrogen gas. This comprises an oxidic support material comprising zirconium dioxide, titanium dioxide, aluminum oxide, silicon dioxide, silicon dioxide-aluminum oxide, zeolites and cerium oxide. Platinum is present as catalytically active metal. Furthermore, alkali metals such as lithium, sodium, potassium, rubidium or cesium can be present as further inorganic compounds so as to improve the activity of the catalyst for removing carbon monoxide by conversion into carbon dioxide in the water gas shift reaction. The catalytically active metal is, according to EP 1 571 125 A2, present in the catalyst in an amount of 2% by weight.
WO 2005/072871 A1 discloses a catalyst for the water gas shift reaction which comprises metallic particles and particles of metal oxide. Suitable metal oxides are, for example, cerium oxide, titanium dioxide, iron oxide, manganese oxide or zinc oxide. Suitable metal particles are, for example, gold or platinum and are present in an amount of from 0.5 to 25% by weight, based on the oxidic material.
US 2006/0002848 A1 discloses a catalyst which has a support material composed of, for example, aluminum oxide, titanium dioxide, silicon dioxide, zirconium dioxide or a combination thereof. Furthermore, alkali or alkaline earth metals and also metals selected from among lead, bismuth, polonium, magnesium, titanium-vanadium-chromium, manganese iron, nickel or cobalt, etc., can be present. Catalytically active metals present are, for example, platinum, palladium, copper, rhodium, etc.
EP 1 908 517 A1 discloses a catalyst for converting H2O/carbon monoxide into hydrogen and the use of this catalyst for increasing the concentration of hydrogen in a stream used for supplying a fuel cell. This catalyst is a solid comprising an active phase comprising elements of group VIII on a support material comprising aluminum oxide, silicon dioxide, zirconium dioxide or mixtures thereof and a promoter from the group of the rare earths, for example lanthanum or cerium.
US 2005/0207958 A1 discloses a process for reducing the amount of carbon monoxide in a water gas shift reactor without formation of methane. A catalyst having a support material based on cerium oxide and zirconium oxide or cerium oxide and lanthanum oxide is used for this purpose. As promoters to avoid methanation, use is made of copper, manganese, iron compounds or combinations. Further promoters can be alkali or alkaline earth metals. The amount of platinum present on the catalyst is at least 1% by weight.
US 2005/0191224 A1 discloses a catalyst for separating off carbon monoxide from hydrogen gas. The catalyst used for this purpose has a support composed of metal oxide and has a platinum component and an alkali metal applied to this support. According to this document, zirconium dioxide, titanium dioxide, aluminum oxide, silicon dioxide, silicon dioxide-aluminum oxide, zeolites or cerium oxide, for example, are suitable as support material.